Physics 327 Lab 2
7/25/16
PHYSICS 327, 2009
LAB 2 - AC INTRO AND VOLTAGE DIVIDER
Goals: Learn about AC voltage divider, differentiator, and practice with oscilloscope.
Please read chapter 17 of Faissler before coming to the lab to familiarize yourself with
oscilloscopes.
Start out with some practice with the oscilloscope and function generator: Hook the
function generator output to the oscilloscope input B, and obtain a trace for the
generator output. Set the frequency to about 1 kHz. On the scope, check that you are
set for DC coupling, dual mode (you will want both channels later), and AC sync. Set
the scope time base to about 1 ms. Adjust the trigger level and find the signal. Change
the various controls on the function generator to see how the signal changes.
An AC voltage divider is shown below:
The circuit is described by the equation V out  Vin
Z2
. Let Z1 be a capacitor and Z2 a
Z1  Z2
resistor, i.e. Z1 = -j/ C and Z2 = R . Set up this circuit with R = 20 k and C = 1000
pF (1 pF=10-12 F) .
Because the impedance Z1 depends on frequency, the output voltage V out will also
depend on frequency. The angular frequency  is related to the frequency f (in units of
Hz) by = 2f. First, use a sinusoidal input voltage, and measure the dependence of
the magnitude and phase of
Vout
relative to
Vin, and compare to
calculations. Measure data points at 100 Hz, 300 Hz, 1 kHz, ... 1 MHz, to get about
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Physics 327 Lab 2
7/25/16
equal spacing in log(f). Graph log(Vout/Vin) vs. log(f), and the phase difference (linear)
vs.
log(f). Calculate how the theoretically expected
Vout/Vin and phase difference
should look as functions of frequency. Compare the theory to data by making graphs of
Vout/Vin and phase difference which combine both the data and the theory.
To investigate the exponential decay time dependence, use the function generator to
make a square wave pulse. For this input to the voltage divider, estimate the time
constant of the exponential decay in Vout by measuring V(t), i.e., voltage as a function of
time, on the scope, and compare to the calculated time constant. Put your Vout
measurements into a Origin plot and add the expected decay curve derived from the
values of C and R.
To see this circuit act as a differentiator, use the function generator to generate a
triangular waveform. Plot both output and input voltages as a function of time on the
oscilloscope screen. How close is Vout to the expected differentiated wave forms?
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